There are presently known many panoramic imaging cameras and optical systems that use a variety of refractive and reflective optical components. Fish-eye lenses, for example, are built strictly from refractive lenses. Fish-eye lenses can cover very wide fields of view, up to 220 degrees. Disadvantages of fish-eye lenses are well known. Among of the disadvantages are non-uniform image illumination and resolution across a wide field of view. A pure refractive optical system, such as fish-eye lens, is commonly called a dioptric optical system.
Another type of panoramic imaging system comprises mirrors and lenses. This type of optical system is called a catadioptric optical system. Catadioptric optical systems usually use one or two concave and/or convex mirrors and one or two relay lenses, usually placed behind the mirrors. Catadioptric systems have been developed to achieve a super-wide-angle field of view.
A simple catadioptric system with one mirror and one relay lens was introduced by James S. Conant (U.S. Pat. No. 2,299,682). The patent describes a convex parabolic or hemispherical mirror which creates a curved virtual image, projected by a camera lens onto a photographic film or plate. In this case, a standard camera lens serves as a relay lens. The main disadvantage of the Conant optical system, as with most wide-angle optical systems, is poor image quality due to uncorrected field aberrations like astigmatism and field curvature.
Reducing the aforementioned deleterious effects produces sharper images. Further, a higher F-stop number is necessary and an exposure time must be increased to maintain image quality for stills and to increase frame rates for video recording.
Significant improvement in the Conant optical system was made by Shree K. Nayar, inventor of a panoramic camera using a convex parabolic mirror with a telecentric relay objective lens and a standard camera lens, which renders a hemispherical scene from a single point of view onto a two-dimensional format (U.S. Pat. No. 5,760,826).
The main disadvantage of the Nayar optical system is an uncorrected field curvature, reducing image quality, particularly in compact optical systems with low F-stops. To get satisfactory sharp images, the F-stop for the Nayar optical system should be set greater than or equal to 8, depending on the sensor format, and as a consequence, greater object illumination and longer exposure times are required. Greater illumination requires using bulky overall packages, as parabolic mirror diameters should be on the order of 3-4 inches, with optical system lengths of about 12 inches.
Another attempt to improve image quality in catadioptric panoramic optical systems with parabolic mirrors was recited by Edward Driscoll, Jr. et al. (U.S. Pat. No. 6,480,229 B1). In the Driscoll optical system, two field-flattening correction lenses are introduced. A first correction lens is positioned between a parabolic mirror and a camera lens; a second correction lens is positioned between the camera lens and a sensor plane. The first correction lens corrects for astigmatism; the second correction lens corrects for field curvature. No detailed description of either correction lens is recited in the Driscoll patent, no data are provided about level of this type of correction or achieved optical image quality. Further, astigmatism and field curvature effects are difficult to separate if the field-flattening lens does not lie in the image plane. The Driscoll optical system is a derivative of the second Nayar optical system, where the first and second correction lenses in the Driscoll optical system replace the Nayar relay lens. Both optical systems improve the image quality of the original Conant optical system, which does not provide for field-flattening correction.
Javaan Singh Chahl, et al. (U.S. Pat. No. 5,790,181), Alfred M. Bruckstein, et al. (U.S. Pat. No. 5,920,376), and Yasushi Yagi, et al. (U.S. Pat. No. 6,130,783) address single-mirror and two-mirror panoramic optical systems without detailed description of optical image formation and optical image quality. Primarily, Chahl, Bruckstein, and Yagi recite principal rays and principle ray mapping onto the sensor plane. Ray bundles and the associated optical aberrations like uncorrected field curvature, astigmatism, and lateral color displacement are not addressed in these three patents. Higher F-stops are necessary for the optical systems represented in each of the three aforementioned patents to sharpen the image, yielding low image illumination levels and poor contrast.
To further correct field curvature in panoramic optical systems, Gottfried R. Rosendahl, et al. (U.S. Pat. No. 4,395,093) introduces a convex hyperbolic mirror used in conjunction with a custom objective lens comprising twenty one lens elements.
Arthur Cox (U.S. Pat. No. 4,484,801), in a similar approach, uses hyperbolic mirror and a custom objective lens containing 16 single lens elements to correct field aberrations and improve image quality. Both custom objective lens optical systems require a high F-stop, require complex optics, and in the end, do not eliminate high image compression.
Digital imaging optics allows for additional freedom in image quality correction, above and beyond that of photographic film optical systems, in the design of an integrated digital sensor and in the function of digital image processing software. High quality digital images, free of image compression and panoramic distortion, can be achieved by using panoramic optics with the integration of the digital sensor and image processing software into the optical system.